New technology allows women to put their eggs on ice. Doctors urge caution, but some people can’t wait.

Sauntering into a coffee shop in Pasadena, Calif., Cassandra McCarthy—pink flip-flops, big smile—looks carefree. But McCarthy, 34, is worried: will she find a mate and have kids before her fertility plummets? A few months ago she Googled the Web and hit on a new company, Extend Fertility. For about $13,000, plus a $500 annual storage fee, doctors would freeze her eggs for later use. In June, McCarthy took out her credit card, signed up and breathed a sigh of relief. “There’s a peace of mind knowing I didn’t leave my fertility to chance,” she says.

For decades, frozen sperm and embryos have created thousands of babies for infertile couples, making young single women with old-fashioned dreams (husband first, then kids) bystanders to the reproductive revolution. Now there’s egg-freezing. While still evolving—only about 100 babies have been born so far—the science, researchers say, has advanced significantly in the last few years. Extend Fertility, launched this spring by Harvard M.B.A. Christy Jones (34 years old, 12 frozen eggs), is now recruiting patients and partnering with fertility centers across the country, from California to New York, to create a nationwide network of egg-freezing clinics. For women who see their fertility nearing its shelf life, egg-freezing is the greatest thing since birth control.

The procedure starts with hormone injections, which increase the number of eggs a woman produces, ideally to about a dozen. The eggs are then extracted, treated with a protectant and submerged into a tank of liquid nitrogen. Years later, a woman has them thawed, fertilized and implanted into her womb. There are numerous challenges. The drugs can cause side effects and don’t always work perfectly, as McCarthy knows well. Her first hormone shot sent her estrogen surging; now she’s waiting to try again. And there are significant technical hurdles. Because the human egg is large and watery, it is highly susceptible to freezer burn. Its delicate spindle apparatus, which divides chromosomes after fertilization, can be damaged during freeze or thaw. And there’s the supply problem: frozen sperm come in batches of millions—lose a few thousand, no big deal. With eggs, there’s no room for error. “It’s not a guarantee,” says Dr. Bradford Kolb of Huntington Reproductive Center, Extend Fertility’s first clinic. “It’s an evolving technology.”

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